1. Field of the Invention
This invention relates to a radio receiver including frequency conversion means for compensating for a DC offset introduced by analog-to-digital conversion of an intermediate frequency (IF) signal generated within the receiver.
2. Description of the Background Art
In radio receivers employing digital modulation, such as PSK modulation (phase shift keying modulation), FSK modulation (frequency shift keying modulation), and ASK modulation (amplitude shift keying modulation), the receiver converts a received signal, which is to be demodulated, into a digital signal and then decodes the digital signal. In general, since the received signal has a high frequency bandwidth (e.g. 800 MHz or 1.9 GHz) at a carrier frequency, the radio receiver removes the carrier from the received signal and converts it to an intermediate frequency (IF) signal having a frequency which is lower than the carrier frequency prior to decoding. The IF signal is then converted from an analog signal into a digital signal by use of an analog/digital converter.
However, the output of the analog/digital converter has a noise component in the form of a direct current offset which is added to the digital signal. The direct current offset has the effect of deteriorating the accuracy with which the digital signal is decoded. As a result, it is necessary to compensate for the direct current offset to assure accurate decoding of the received signal.
In a conventional method of compensating for the direct current offset, the radio receiver measures the magnitude of the direct current offset. Based on the result of this measurement, the receiver controls and adjusts the level of the digital signal. Specifically, the receiver compensates for the direct current offset in the digital signal by using a well-known feedback loop control. However, it is difficult and time consuming to design and adjust the feedback loop since the designer of the circuit must take into account the stability and response time thereof. Further, since, the feedback loop control circuit is complicated, a radio receiver having a feed back loop is relatively large in size and consumes relatively large amounts of power.